The present disclosure relates to a strengthened glass. More particularly, the disclosure relates to chemically strengthening glass by ion exchange. Even more particularly, the disclosure relates to chemically strengthening of glass by multiple ion exchange treatments.
Glasses may be chemically strengthened by an ion exchange process. In this process, metal ions that are present in a region at or near the surface of a glass are exchanged for larger metal ions, typically by immersion of the glass in a molten salt bath. The presence of the larger ions in the glass strengthens the glass by creating a compressive stress in a region near the surface. A tensile stress is induced within a central region of the glass to balance the compressive stress. If the tensile stress is excessive, the glass either becomes frangible—i.e., energetically fragments into a large number of small pieces—or crazes—i.e., cracks without the cracks penetrating the glass.
Chemically strengthened glasses have recently been identified for use in hand held devices, such as mobile phones, media players, and other devices, as well as other applications requiring transparency, high strength and abrasion resistance. For such applications, however, a glass that is either frangible or crazes upon impact is undesirable.